1. Field of the Invention
The invention relates to an optical proximity correction (OPC) method, and more particularly, to an OPC method by sequentially performing a rule-based OPC process and correcting a specific photomask pattern manually.
2. Description of the Prior Art
For transferring a pattern of integrated circuits to a semiconductor substrate, the manufacturer has to produce a photomask with a photomask layout of the circuit design, and then to perform a lithography process to expose the patterns of the photomask layout to a semiconductor substrate in a predetermined ratio. As a result, a lithography process is one of the most important processes in semiconductor fabrication.
The critical dimension (CD) of the designed photomask pattern is limited by the resolution limit of the optical exposure tool, so that an optical proximity effect (OPE) easily occurs when high-density photomask patterns of high circuit integration are exposed to the semiconductor substrate. OPE causes deviations when transferring the photomask patterns. For example, right-angled corner rounding, line end shortening, and line width increasing/decreasing are common defects caused by OPE.
To prevent the defects of the photomask pattern caused by OPE, an OPC process for correcting the photomask layout is usually preformed when fabricating the photomask. The conventional OPC process includes a model-based OPC process or a rule-based OPC process. The rule-based OPC process uses a correction rule of a database to correct the original photomask pattern by taking account of the width and spacing of the original photomask pattern. The model-based OPC process comprises exposing a test photomask to compare the result with the original photomask pattern and using a simulation tool to make complicated calculations. Generally, a large amount of time is required to make the comparison, calculation, and simulation of the test photomask pattern. Therefore the model-based OPC process is not efficient, even though it can obtain a better correction. On the other hand, the rule-based OPC process can compute the target bias to correct the photomask layout faster, but the rule-based OPC may have greater errors resulting from interpolation and unsuitability of functions so that the reliability of the corrected photomask layout is decreased.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of an original photomask layout 10 of the prior art. The photomask layout 10 comprises a first photomask pattern 12 and a second photomask pattern 14. The first photomask pattern comprises a first straight line 12a, a second straight line 12b, a third straight line 12c, and a fourth straight line 12d arranged in parallel with each other, wherein a line-end of the second straight line 12b connects to the second photomask pattern 14. If the original photomask layout 10 is directly used to produce on a photomask for performing a lithography process, a serious OPE will occur so that the exposed pattern in a photoresist layer on a semiconductor substrate will have deviations and defects. Therefore an OPC process has to be performed on the original photomask layout 10.
Referring to FIG. 2, FIG. 2 is a schematic diagram of a corrected photomask layout 16 of the original photomask layout 10 shown in FIG. 1 according to a conventional rule-based OPC process, wherein the dotted lines denote the exposed pattern in a photoresist layer. In the corrected photomask layout 16 according to the rule-based OPC process, an assist feature 18 that can avoid a right-angled corner rounding situation or a split in the straight line resulting from the OPE is added to the line-end closer to the second photomask pattern 14 of the second straight line 12b so that no serious defects occur at the intersection of the second straight line 12b and the second photomask pattern 14. However, in the corrected photomask layout 16 according to the conventional rule-based OPC, as shown in FIG. 2, the defects of line-end shortened still exist in the first, the third, and the fourth straight lines 12a, 12c, and 12d. More seriously, a “bridge problem” occurs at the intersection of the second straight line 12b and the assist feature 18, which means a weak point PW looking like a bridge or a neck shape appears in the photoresist layer resulting from the light scattering ability or the light reflectivity of materials. This problem often occurs in the second straight line 12b with longer length, and the location of the weak point PW is horizontal with the line-ends of the first straight line 12a and the third straight line 12c near the second straight line 12b. As shown in FIG. 2, the weak point PW of the second straight line 12b is very narrow. This will probably cause a defect such as a broken circuit in following production processes. In a more serious situation, an opening of the second straight line 12b may be present at the weak point PW, resulting in that the whole product has to be abandoned.
To conclude the above description, the prior-art rule-based OPC process still cannot correct a photomask layout to obtain an ideal transferred pattern in the photoresist layer effectively. Although the prior-art model-based OPC process can generate a better correction result, it takes a huge amount of time to perform modeling calculations and tests, which cannot match the requirements of a factory of being highly efficient and low cost. As a result, to correct the photomask layout effectively with a simple solution is still one of the important issues in semiconductor manufacturing.